This invention relates to testing apparatus for internal combustion engines and more particularly relates to apparatus for displaying voltage waveforms derived from the ignition systems of internal combustion engines.
Vehicular ignition systems basically consist of a battery, a distributor including primary and secondary coils, means for periodically commencing and terminating the conduction of current through the primary coil so that a high voltage is induced in the secondary coil and a spark plug for each individual cylinder of the vehicular engine. The distributor is connected to each spark plug by a wire or lead.
One way to diagnose the system's condition is to study the voltage waveforms at various points in the system. The waveforms are usually coupled from these points and displayed on a cathode ray tube of a conventional oscilloscope. One of the most useful waveforms is generated by the secondary coil of the distributor. In the past, the secondary coil and distributor have been manufactured as individually separate units. The secondary coil is normally connected to the distributor cap by a high tension lead.
In such a system, there are two common methods for obtaining a display of the secondary coil voltage waveform:
The first method is to insert a coupling transformer in series with the high tension lead between the secondary coil and the distributor cap. This is accomplished by removing the high tension lead from the coil tower. A specially constructed coupling transformer is then inserted into the coil tower, and the high tension lead is reinserted into the coupling transformer to complete a series circuit between the secondary coil and the distributor. The voltage waveform from the coupling transformer is coupled to an oscilloscope and is displayed.
The second method uses a capacitive-type pick-up which fits around or clamps onto the high tension lead. A typical pick-up of this type is illustrated in U.S. Pat. No. 3,477,024 (Pelta -- Nov. 4, 1969). Such a probe operates on the principal of a capacitor and has two opposed metallic plates which are separated by the high tension lead when the device is properly positioned. The opposed plates of the pick-up are capable of responding to time-varying electric fields like a conventional capacitor.
Recently, automobile manufacturers have developed a "unitized" ignition distributor in which the distributor primary and secondary coils are integrally formed within the distributor cap. As a result, there is no external high tension lead between the primary and secondary coils and the distributor cap. Therefore, neither a series coupling transformer nor a clamp-on capacitive pick-up are useful.
Accordingly, it is a primary object of the present invention to provide a sensor capable of responding to the time-varying electrical phenomenon produced by the secondary coil of a distributor without employing either a transformer or a capacitive pick-up.
It is another object of the present invention to provide a sensor of the foregoing type in which no direct access to the conductor connecting the secondary coil to the distributor cap is required.
It is still another object of the present invention to provide a probe of the foregoing type which produces voltages capable of being displayed on an oscilloscope.